This invention relates generally to high barrier laminates and more specifically to laminates useful in thermoforming operations.
Articles such as food items and more specifically meat products are often packaged in thermoplastic films or laminates in order to protect the product to be packaged from exterior abuse and environmental contamination, and to provide a convenient and durable package for distribution of the product and display in a display case or other point of sale. When dealing with meat products in particular, it is usually desirable to provide a film having good oxygen barrier characteristics, in order to reduce the infiltration of oxygen into the meat product with its detrimental effect on freshness, color, and other properties of the meat product.
Thermoforming methods such as vacuum forming or plug-assist vacuum forming are often useful in packaging meat products. In general terms, thermoforming involves heating of a thermoplastic film or laminate and forming the film or laminate into a desired shape for holding a meat product to be inserted. This formed sheet of a film or laminate is usually referred to as a forming web. Various systems and devices are used in a thermoforming process, often accompanied by vacuum-assist and plug-assist components to provide the proper forming of the forming web into a predetermined shape. Thermoforming systems are well known in the art.
Although many types of films and laminates have been utilized in thermoforming operations, including shallow draw operations in which the forming web takes on a shallow shape i.e. a relatively low profile form, it has been found that certain problems may arise during the thermoforming step. One such problem is known as "line draw".
This phenomenon may be described as an extreme thinning of thermoplastic crystalline material in a direction at 90.degree. to the direction of orientation of the material. Lines of thinned, stretched material are observable, usually running parallel to the oriented direction of the film (usually the machine or longitudinal direction).
Line draw is an undesirable characteristic of a thermoplastic crystalline material which has been produced with either substantially no orientation, or orientation principally in one direction only, usually the machine direction. Upon subsequent thermoforming of the film, the film is stretched as it is shaped to the thermoformed mold configuration. In the unoriented direction, the material will elongate at the yield point, resulting in elongated tapered streaks aligned in the oriented direction.
This phenomenon results in reduced strength at the thermal areas, and a mottled appearance.
It has now been discovered that by orienting a film to substantially the same extent in both the longitudinal and transverse directions, at relatively low maximum orientation ratios, a film having good thermoforming properties with little or no line draw is obtained.
It has also been discovered that such balanced orientation, at relatively low orientation ratios, achieves a film especially useful in shallow draw thermoforming operations.
Of interest is U.S. Pat. No. 4,399,180 issued to Briggs et. al. and disclosing the use of a primary layer of linear low density polyethylene with at least one coextruded layer on one side thereof of a highly branched low density polyethylene, the laminates being useful in stretch wrapping around pallet loads and the like.
Also of interest is U.S. Pat. No. 4,425,268 issued to Cooper and disclosing a stretch wrap film comprising about 40 to 90 percent by weight of ethylene vinyl acetate and about 8 to 55 percent by weight of a linear copolymer of ethylene and a higher alkene.
Also of interest is U.S. Pat. No. 4,364,981 issued to Horner et al disclosing a blown coextruded tubular film of three layers, with a core layer of low pressure low density polyethylene and outer layers of high pressure low density polyethylene.
Of interest is U.S. Pat. No. 4,457,960 issued to Newsome and disclosing multi-layer structures including linear low density polyethylene in at least one layer, and blends of linear low density polyethylene with ethylene vinyl acetate copolymer. The multi-layer structures may be biaxially oriented.
It is an object of the present invention to provide a film or laminate suitable for use as a forming web in a thermoforming process.
It is a further object of the present invention to provide a film or laminate useful in thermoforming operations, yet avoiding the line draw problem.